Researchers discover star in closest known orbit around black hole

An international team of astronomers has observed evidence of a star that whips around a black hole at a rate of nearly twice an hour. If confirmed, the finding could demonstrate the tightest orbital dance between a black hole and a companion star ever seen.

The study is scheduled to appear in an upcoming issue of Monthly Notices of the Royal Astronomical Society and is now available online.

The discovery was made using the collective power of three of the most advanced X-ray and radio telescopes in existence – NASA's Chandra X-ray Observatory; NASA's Nuclear Spectroscopic Telescope Array (NuSTAR), which observes higher-energy X-rays; and the Australia Telescope Compact Array (ATCA), a state-of-the-art set of six radio telescopes operated by the Astronomy and Space Science Division of the Commonwealth Scientific and Industrial Research Organisation (CSIRO).

"Combining the unrivaled capabilities of the three telescopes through simultaneous observations led us to the discovery of the highly unusual nature of this stellar couple," said Slavko Bogdanov, associate research scientist in the Columbia Astrophysics Laboratory at Columbia University and a co-author on the paper.

The compact, closely bonded stellar couple – known as a binary – is located in the globular cluster 47 Tuc, a dense cluster of stars in our galaxy about 14,800 light years from Earth in the southern constellation Tucana.

While astronomers have observed this extraordinary binary, called X9, for many years, it wasn't until 2015 that radio observations revealed the pair likely contains a black hole that pulls material from a companion star called a white dwarf, a low-mass star that has exhausted most or all of its nuclear fuel and has essentially burned out.

While analyzing data collected about two other, unrelated X-ray binaries, Bogdanov and co-researchers using the Chandra X-ray Observatory realized the data could be utilized to further study the peculiar X9 system. Researchers subsequently decided to combine the superb capabilities of the three telescopes to study the relatively unexplored binary in unprecedented detail.

The resulting Chandra data of X9 reveals that it changes in X-ray brightness in the same manner every 28 minutes, which is likely the length of time it takes the companion star to make one complete orbit around the black hole.

"We see the brightness vary on a 28-minute timescale, for an as-yet unknown reason. The regularity of the variation suggests that this is associated with the orbital period," Bogdanov said. Chandra data also shows evidence of large amounts of oxygen in the system, which is characteristic of white dwarfs, he explained. The researchers believe this indicates the companion star is a white dwarf, which would then be orbiting the black hole at only about 2.5 times the separation between the Earth and the Moon.

"Due to the proximity of the white dwarf to the black hole, the immense gravitational pull of the black hole rips off matter from the surface of the star," Bogdanov added. "This matter accumulates in a disk of matter before spiraling in past the black hole event horizon, never to be seen again. While it is unlikely that the entire white dwarf will be devoured by the black hole, it is not entirely clear what its ultimate fate will be."

"Eventually so much matter may be pulled away from the white dwarf that it ends up becoming an exotic kind of planet," said co-author Craig Heinke, of the University of Alberta. "If it keeps losing mass, the white dwarf may completely evaporate."

While it remains unknown how the tight pairing initially formed, there are several strong possible explanations. The most plausible theory is that the black hole collided with a red giant star, a highly conceivable scenario since the binary exists in a globular cluster, where stars are close enough to each other that they interact gravitationally and have opportunities to crash into each other. If such a collision occurred, gas from the outer regions of the star would have been ejected from the binary. The remaining core of the red giant, the researchers believe, would have formed into a white dwarf, which would in turn have become a binary companion to the black hole. The orbit of the binary would then have shrunk as gravitational waves were emitted, until the black hole started pulling material in from the white dwarf.

An alternative explanation for the observed behavior of X9 is that the white dwarf's partner is a rapidly spinning neutron star instead of a black hole. In this scenario, the neutron star spins faster as it pulls material from a companion star via a disk, a process that can lead to the neutron star spinning around its axis thousands of times every second. This possibility is less likely based on the extreme variability seen from the X-ray and radio observations; however, the researchers cannot yet disprove this explanation and plan to continue studying X9 to better understand the properties of such extreme systems.